Example 2 (Co-authorships)  
A popular and seminal model in the economic literature on networks is the “Co-author model” of [1]}  and later extended by [2]}:With the notation used here, the model by [1]} and by [2]} consider activities that are performed only by pairs of agents, as stated in Assumption REF . agents are researchers and links are pair-wise collaborations on scientific projects, which are costly but provide payoffs that depend endogenously on the negative externalities given by each collaboration to the other co-authors of an author.
